SENS

Student-Engineered Nanomotion Sensor

 MS Word Version

 

 

Colorado School of Mines

EPICS-151 Fall 2002

November 15, 2002

 

SeisMasters Design Team:

 

Vincent Michael Gonzales

Robert T. Wagner

Michael Tyndall

Kyung Lim

Brian Law


Executive Summary

 

The SeisMasters design team has designed an inexpensive, long period seismometer entitled “SENS” (Student-Engineered Nanomotion Sensor) in accordance with the requests of clients John Lahr and Thomas Boyd. The clients mandated the seismometer must be able to detect earthquakes that occur anywhere in the world of magnitude 6.0 or greater, must be easily constructed, must be compatible with AmaSeis software, and cost less than $150 to build. The seismometer will be used in K-12 classrooms to promote hands on learning experiences. In the future, the seismometer could possibly be used internationally with the help of an endorsement by the GLOBE program.   

 

At the onset of our designing process, we chose to deviate from common designs of vertical and horizontal seismometers (particularly Lehman). Our reasoning behind this decision was that we wanted to produce a design that would detect motion in any direction. Additionally, we wanted our seismometer to be unique when compared to seismometers designed by our peers. After careful analysis, we decided to focus on a seismometer based on an inverted pendulum. This chosen design offers advantages in cost, sensitivity, and ease of construction.

 

Our seismometer is based off movement by an inverted pendulum made from a hand coiled spring, a rod, and a magnet. The spring rests upright on a plastic base and has a brass rod attached vertically to the top of it. Sitting atop of the brass rod is a relatively small but powerful magnet. The current from magnetic induction is run through an amplifying circuit so that even the smallest movement of the pendulum will produce a readout through the AmaSeis software.

 

The structural components of our seismometer consist of a PVC pipefitting and a 2-litre pop bottle. The pipefitting acts as the base of out instrument while the 2-litre bottle is modified to support the hanging coil and prevent air currents from acting on the pendulum.

 

Its straightforward design makes the SENS seismometer inexpensive and easy to build, yet highly sensitive. The design meets or exceeds all of the project parameters.

 

Product Overview
Table of Contents

 

1.0  Problem Statement                                                                                                          4

1.1  Goals

1.2  Guidelines

1.3  Important Considerations   

2.0  Preliminary Ideas                                                                                                             4

2.1  Design Options

2.2  Design Choice

3.0  Final Design                                                                                                                     6

3.1  Pendulum                                                                                                               6

3.1.1        How it Works

3.1.2        Specifications

3.2  Base/Frame                                                                                                            6

3.2.1        General

3.2.2        Specifications

3.3  Coil                                                                                                                        7

3.3.1        General

3.3.2        Specifications

3.4  Analog to Digital Converter                                                                                     8

3.4.1        Information

 

4.0  Analysis                                                                                                                            8

4.1  Performance

4.2  Assembly

4.3  Cost

 

5.0  Final Ideas                                                                                                                        9

6.0  References                                                                                                                       10

7.0  Assembly Instructions                                                                                                     11


1.0   Problem Statement

 

1.1  Goals

The SeisMasters Design Team has endeavored to design a product that fully meets the requested parameters. During this process we have kept these goals for our 
project in mind:
 
·        Advance the learning experience of students through hands on activities.
·        Remove financial barriers to amateur seismology.
·        Develop a product that could eventually have an international impact through endorsement by the Globe program.

 

1.2  Guidelines

As requested by the clients, Thomas Boyd and John C. Lahr, the SeisMasters Design Team has developed an inexpensive sensor entitled “SENS” (Student-Engineered Nanomotion Sensor) for seismic waves. Mandated product guidelines were considered throughout every step of the design process: the seismometer must be a PC-based instrument that costs less than $150 to build. Its design, construction, and operation must be compatible with the skill set and knowledge of K-12 students and teachers. The instrument must be sensitive enough to detect long period seismic activity from an earthquake occurring anywhere in the world of magnitude 6.0 or greater. Output format is limited to either the serial or game port format of a PC (via an analog-to-digital converter), as data will be recorded by AmaSeis software. 

 

1.3  Important Considerations

From the beginning of the design process we realized that it would be necessary to prioritize our project’s target parameters. Of paramount concern was the sensitivity and reliability of the seismometer. Another goal held in high regards was the production of an instrument that would cost, at most, $150 and less if possible. Of subjacent concern was our desire to produce an innovative design that would stand out from those of our peers. It was with these goals in mind that we developed our design.

 

2.0  Preliminary Ideas

 

2.1  Design Options

The most common design type for inexpensive seismometers is the Lehman horizontal pendulum (fig. A). Its simple construction has made it popular amongst amateur seismologists. However, it lacks the foundation in mechanics that a more solid design should have. The major problem with this design is that an undamped pendulum with a period of 10s or greater would be prohibitively long (in excess of 100m) [1]. A sensor with a period below the desired range will not achieve the resonance needed for the sensitivity specifications required. A similar but less common application detects vertical motion, with the use of a horizontal arm (fig. B) [2]. Unfortunately, this design also delivers an insufficient period and only senses one direction of motion.

 

 

 

          

Fig. A – Horizontal Pendulum Seismometer     Fig. B – Vertical Sensor Design

www.infiltec.com/seismo/ - 11/14/02             www.infiltec.com/seismo/ - 11/17/02

 

 

When initially considering inverted pendulums as a design, we discussed two variations. The first used an adjustable electromagnet to attract a conducting mass at the end of a hinged rod. In order to achieve simple harmonic motion with this kind of a pendulum, the size ratio of the electromagnet to the pendulum radius would have to be very large. This creates practicality problems: either the electromagnet would need to be so large that a very high current would be required to generate a sufficient magnetic field, or the pendulum would need to be very small to the point where it would be difficult to work on. The second variation consists of a simple magnet and rod attached atop a spring.

 

2.2  Design Choice

With the above factors in mind, the design type we chose will have a permanent magnet attached the end of a rod held vertical by a spring (fig. C). This method boasts the most simple and economical construction of the methods discussed. Unlike horizontal pendulum designs, the vertical spring will effectively detect lateral motion in any direction (as opposed to just one – fig. D). This unique quality optimizes the amount of seismic data we can record.

 

                              

                  Fig. C – Pendulum Detail                      Fig. D – Movement Detail

                  Robert Wagner – 11/22/02                   Vincent M Gonzales 11/22/02

Our design would be constructed from easily attainable components and can be assembled using common tools. The device, as foreseen, would also be very sensitive to ambient air movement, however its size and shape make it easy to shield. In our design we used a common two-liter bottle. During an interview with Dr. Todd Ruskell, a professor of physics at CSM, he expressed confidence in the mechanics of our design as well as interest in our unique approach [3]. 

 

As explained, this design offers ease of construction, and sensitivity over other models. It is, in our opinion, the best synthesis of all the required project elements.

 

3.0  Final Design

 

3.1  Pendulum

3.1.1        How it Works

The inverted pendulum consists of three major parts: a coiled spring, a metal rod, and a magnet (fig C). When seismic activity occurs in the earth’s surface, the harmonic motion of the ground will send the pendulum system into oscillation.

 

3.1.2        Specifications

The spring is hand wound from 19-gauge steel wire supplied with the kit. The spring, fabricated by the user via supplied instructions, will have a spring constant that will be just strong enough to hold the pendulum upright (this property optimizes the desired period). The optimal spring design was found by testing a number of different wire types and coil configurations.

 

The spring will be formed by winding 19 gauge wire around a .75” x 4” wooden dowel. The spring will have approximately 7 turns and be 1.0”-1.5” high. The top and bottom of the wire spring will be secured with epoxy to the rod and plastic base, respectively.

 

The rod is made of brass and has a diameter of 0.125” and a length of 4”. Brass was chosen over steel so that the magnetic field emitted by the magnet would not be disrupted. Also, brass is less expensive than steel.

 

The magnet is a round 0.50” x 0.50” cylindrical nickel-plated neodymium magnet. The neodymium magnet was chosen over common ceramic magnets for its remarkable magnetic strength at a comparable price.

 

 

3.2  Base/Frame

3.2.1        General

The frame of the system will connect all mechanisms of the seismometer. It is composed of a common 2-liter bottle and a plastic base. A PVC pipe fitting available at most hardware stores will serve as the base. The 2-litre bottle will fit snugly over the plastic base (fig E). A 2-liter bottle was chosen for the frame because it is inexpensive, easy to find, simple to work with, and can serve as both a transparent wind shield and a structural support.

 

Fig E. Product Overview

Robert Wagner – 11/22/02

 

3.2.2        Specifications

The plastic base is a NDS E-Z channel end cap / end outlet (UPC: 0 52063 81813 9). The inside diameter of the pipe fitting matches that of the pop bottle at 4.25", allowing a snug fit. The 2-liter bottle will have 2.75” cut off of the bottom prior to assembly.

 

3.3  Coil

3.3.1        General

The inductance coil will be suspended from the top of the pop bottle by a brass rod. Attached to the bottom end of the rod is a plastic sewing machine bobbin wrapped with enameled copper wire. A brass suspension rod and a plastic sewing bobbin were chosen for their non-magnetic properties.

 

3.3.2        Specifications

The copper wire will be wrapped around a .5" high x .5" wide sewing bobbin. The bobbin has an inner diameter of .25". The 4” rod will hang from the top of the 2-liter bottle cap through a drilled hole, secured by epoxy.

 

We have selected 34-gauge enameled copper wire for the coils because it has a diameter that is small enough to permit approximately 1000 turns on our bobbin. About 150' of the wire will be needed. Both ends of the coil wire will be run through a second hole in the top of the 2-liter bottle cap to make connections with the amplifier circuit.

 

3.4  Analog to Digital Converter

3.4.1        Information

In order to achieve an interface compatible with a personal computer running AmaSeis software, the seismometer output must be run through an analog to digital converter. The Dataq Company sells a 12-bit unit for $100 that has been recommended by clients John C. Lahr of the US Geological Survey and Thomas Boyd of the CSM Department of Geophysics [4]. The unit converts the signal to a format that can be analyzed by AmaSeis.

 

4.0  Analysis

 

4.1  Performance

The seismometer design is such that the seismic activity due to a magnitude 6.0 of greater earthquake anywhere in the world (such that local amplitude > 1.81e-2mm, 10s<period<20s) should be detected.

 

Our primitive seismometer testing resulted in clear graphic patterns in AmaSeis recordings from even the slightest simulated seismic occurrences. Modifications to AmaSeis software would help, as it is currently not optimized for use with seismometers other than the AS-1.

 

4.2  Assembly

The seismometer has been designed so that a middle/high school student or teacher could assemble it within 2 hours. Tools required for construction are: small drill, wire cutters, and a knife or razor blade.

 

4.3   Cost overview

 

Supply Costs

Price Each

Quantity

Price * Quant

Source

150' 34 gauge enameled copper wire

$1.79

1

$1.79

Electronics Store

.5" x .5" cylindrical neodynium magnet

$3.00

1

$3.00

wondermagnets.com

8" x .125" diameter brass rod

$1.92

1

$1.92

Hardware Store

.5" x .75" plastic sewing bobbin

$0.29

1

$0.29

Hardware Store

24" 19 gauge steel wire

$1.79

1

$1.79

Hardware Store

NDS E-Z channel end cap

$3.19

1

$3.19

Hardware Store

.5 oz quick set epoxy

$2.79

1

$2.79

Grocery Store

2-litre pop bottle

$0.89

1

$0.89

Grocery Store

.75" x 4" or longer wooden dowel

$1.79

1

$1.79

Hardware Store

DI-154RS A/D converter

$100.00

1

$100.00

The Dataq Company

Total

 

 

$117.45

 

 

 

 

 

5.0  Final Solution

Our design will have a permanent magnet attached the end of a rod held vertical by a spring (fig. C). Seismic activity will create movement, and consequentially, oscillation in the rod system, which will be detected by a magnetic inductance coil. The inductance coil will be attached to the serial port of a PC though an amplifier circuit and an analog-to-digital converter.

 

Our design boasts simplicity, economical construction, and ease of use without compromising sensitivity. The SENS project fulfills the requirements of our clients. The SeisMasters Design Team fully recommends our product for widespread K-12 use as well as endorsement by the GLOBE project.

 


 

 6.0 References

 

[1]        Seismic Waves -  http://www.gps.caltech.edu/~polet/body_waves.html

            (10/24/2002 – date accessed).

 

[2]        How to Build an Inexpensive Seismometer (11/6/2002)

http://www.infiltec.com/seismo/

(10/21/02 – date accessed).

 

[3]        Interview with Dr. Todd Ruskell, Professor of Physics, Colorado School of Mines, (11/6/2002).

 

[4]        John C. Lahr and  Thomas Boyd - Client Letter

http://blackboard.mines.edu/courses/1/fall2002-EPIC151CD/content/_8367_1/USDOI__Epics_Fall_2002.pdf - 8/20/02.

 

[5]        How to Build an Inexpensive Seismometer (11/6/2002)

http://psn.quake.net/lehmntxt.html (10/28/02 – date accessed).

 

 

BACK